CRL to Gestational Age Calculator
Convert your baby's crown-rump length from a scan report into an accurate gestational age and due date. Uses the Robinson & Fleming formula — the clinical standard adopted by NICE, RCOG and ISUOG.
Convert CRL to gestational age
Enter the crown-rump length from your ultrasound report.
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Your CRL measurement in context
Where your baby's crown-rump length sits on the full clinical range.
Accuracy of CRL-based dating
A dating scan using crown-rump length is accurate to within three to five days. This is the clinical gold standard for pregnancy dating between 7 and 13 weeks — more precise than any date-based calculator.
Typical CRL by gestational age
How CRL progresses through early pregnancy. Your measurement is highlighted.
Your key upcoming milestones
What should you book next?
Use the Scan Timing Calculator to see which scans are due for your stage — with exact booking windows and direct links.
What is crown-rump length?
Crown-rump length (CRL) is the straight-line measurement from the top of your baby's head to the bottom of their rump — the "sitting height," essentially. It's the single most accurate parameter for dating a pregnancy between about 7 and 13 weeks, because in that window the embryo grows at a very predictable rate. Every millimetre of length maps closely to a specific gestational age.
During a dating scan, the sonographer carefully measures CRL using calipers on the ultrasound image. The measurement is taken when the baby is in a neutral position — neither curled up nor stretched out — and the image shows a clear mid-sagittal view. From that single number, the scan software calculates gestational age to within three to five days. This level of precision is why NICE recommends ultrasound dating in the first trimester for every pregnancy.
This calculator uses the Robinson & Fleming formula — the mathematical relationship between CRL and gestational age published in 1975 and still the international clinical standard. The formula is valid between CRL 5mm (6 weeks 2 days) and CRL 84mm (14 weeks 0 days). Outside this range, other measurements like head circumference and femur length take over as more accurate markers.
Why dating by CRL is so accurate
Before 14 weeks, every embryo grows at very nearly the same rate — genetic, nutritional and environmental variation hasn't yet pushed their sizes apart.
This is why a CRL of 30mm consistently means a gestational age of around 9 weeks 6 days across virtually all pregnancies. Later in pregnancy, babies grow at increasingly different rates, so measurement-based dating becomes less accurate.
When CRL-based dating works best
The Robinson & Fleming formula is accurate between CRL 5mm and 84mm — but there's a sweet spot where it's at its most reliable.
| CRL range | Gestational age | Accuracy | Notes |
|---|---|---|---|
| Below 5mm | Before 6w 2d | Not reliable | Formula unstable; use yolk sac or gestational sac instead |
| 5 – 10mm | 6w 2d – 6w 6d | ±4 days | Valid but at the lower margin of accuracy |
| 11 – 50mm | 7w 0d – 11w 4d | ±3 days | Peak accuracy — ideal dating window |
| 51 – 84mm | 11w 5d – 14w 0d | ±5 days | Still valid but accuracy reduces as growth rates diverge |
| Above 84mm | After 14w 0d | Not applicable | Switch to biparietal diameter (BPD) and head circumference |
Why your CRL-derived dates take precedence
If your dating scan disagrees with your LMP by more than five days, the scan wins. Here's why.
Dating based on your last menstrual period (LMP) assumes regular cycles, precise recall of the period start date, and ovulation on day 14. For many women at least one of those assumptions doesn't hold — irregular cycles, hormonal contraception stopped recently, or simple forgetfulness. LMP-based EDD is accurate to about a week at best.
A dating scan sidesteps all of that. It measures your baby directly and applies a formula derived from tens of thousands of pregnancies. That's why NICE guideline NG201 recommends that if an early ultrasound shows a gestational age different from the LMP-based estimate by more than five days, the pregnancy should be re-dated from the scan.
Your CRL-derived gestational age is therefore your clinical gestational age going forward. Every future scan window — nuchal translucency, NIPT, anomaly scan, growth scans — is calculated from this scan-based date, not your LMP. If your booking appointments were scheduled using LMP, they may need adjusting.
The 5-day rule
If your dating scan gestational age and your LMP-based age disagree by more than 5 days, clinical practice is to re-date the pregnancy from the scan.
Less than 5 days difference: original LMP date usually stands.
More than 5 days: scan date replaces LMP date for all future clinical decisions.
Frequently asked questions
What is crown-rump length (CRL)?
Crown-rump length is the straight-line distance from the top of the baby's head to the bottom of the spine. It's the standard measurement used to date pregnancies between about 7 and 13 weeks, because during this window the embryo's growth rate is remarkably consistent across almost all pregnancies.
Why does my scan report show a different gestational age than my LMP-based calculation?
LMP-based calculations assume regular 28-day cycles and precise recall of your period start date. Real cycles vary. A dating scan measures the baby directly and is accurate to within three to five days. If the two differ by more than five days, NICE guidance says the pregnancy should be re-dated from the scan — the scan-based age becomes your official gestational age going forward.
What formula does this calculator use?
The Robinson & Fleming formula, published in the British Journal of Obstetrics and Gynaecology in 1975. The specific calculation is: gestational age in days = 8.052 × √CRL(mm) + 23.73. This remains the international clinical standard adopted by NICE, the Royal College of Obstetricians and Gynaecologists, and the International Society of Ultrasound in Obstetrics and Gynaecology (ISUOG).
My CRL is below 5mm. Why won't the calculator work?
Between about 6 weeks and 6 weeks 2 days, CRL is very small and the formula becomes unstable — a 1mm variation can shift the calculated age by several days. At this stage, the gestational sac diameter or yolk sac presence are more reliable indicators. An early pregnancy scan at 6–7 weeks will often be more informative than trying to date by CRL this early.
My CRL is above 84mm. What now?
Beyond 14 weeks, CRL loses its dating reliability because babies start growing at increasingly different rates. After this point, sonographers use biparietal diameter (BPD), head circumference, and femur length instead. If you're past 14 weeks and need dating confirmation, a dating scan using these parameters is the appropriate next step.
How accurate is CRL dating for twin pregnancies?
Identical to singletons. Each baby is measured individually, and each will have its own CRL. If twin CRLs differ by more than a few millimetres, the sonographer will investigate further. Identical twins should have nearly identical CRLs; non-identical twins can have small natural variations.
Can I use this if my scan used a different formula?
Yes. The Robinson & Fleming formula is the most widely used and produces results within 1–2 days of other common formulas (Hadlock, INTERGROWTH-21st, etc.) in the CRL range. Your scan report's gestational age and the calculator's output should agree to within a day or two. If they disagree significantly, double-check the CRL figure you entered — it's a common source of error.
What if my scan was last week or last month — can I still use this?
Yes. The scan date field accounts for the gap between when the measurement was taken and today. The calculator derives the gestational age at the time of the scan, then adds the days elapsed to give your current gestation. Your estimated due date stays fixed regardless of when you use the calculator.
Get accurate dating with a private ultrasound scan
Our sonographers perform dating scans between 7 and 14 weeks — same-week availability at our Kensington clinic. Your CRL measurement and gestational age, on a detailed report, within minutes.